Technical Field of the Invention
[0001] The present invention is in the technical field of internal boiler chemical treatment
program controls, and in particular the control of pH/phosphate treatment programs.
Background of the Invention
[0002] A boiler is a vessel in which water is vaporized into steam by the application of
heat, typically on a continuous basis. The steam generated is most often used either
as a direct or indirect heat transfer medium and/or to generate electric power. High
pressure and/or high capacity boilers generally are water-tube boilers in which water
is circulated within tubes and the applied heat (combustion products such as flame
and hot combustion gases) flows across the outside of the tubes. Some of these water
tubes may comprise the walls of the furnace in which the heat-generating combustion
occurs.
[0003] A boiler is a dynamic water system. Essentially pure H₂O leaves the boiler as steam.
Water also leaves the boiler as blowdown. The water lost from the boiler as steam
or blowdown is replenished with boiler feedwater.
[0004] A boiler's internal surfaces are metallic heat transfer surfaces. The formation of
scale deposits on, and the corrosion of, these internal surfaces are undesirable phenomena
in any type of boiler. Measures taken against scale deposits and/or corrosion depend
on the characteristics of the boiler and the boiler feedwater. The preventative measures
are generally the purification of the feedwater, internal chemical treatment(s) of
the water and the discharge of impurities in the blowdown. The major remedial measure
is costly downtime for the cleaning of the internal surfaces. Scale deposition decreases
heat transfer rates and promotes corrosion and overheating of these surfaces, and
thus control of scale deposition is the primary objective of internal chemical treatment
programs such as phosphate residual programs (with and without polymer and/or dispersants),
all-polymer treatment programs, chelation programs and coagulation (carbonate) programs.
These treatment programs provide preferential precipitation and/or solubilization/dispersion
of calcium species and magnesium species.
[0005] Some boilers require the use of feedwater having a high degree of purity to ensure
high operational reliability. The impurities or chemical species that might be present
in boiler feedwater are removed with the use of demineralizers, mixed bed polishers
or other techniques in order to purify the feedwater prior to introduction into the
boiler. The purity standards allow a maximum feedwater hardness of only 0.1 ppm or
less. Such boilers are often, but not necessarily, high pressure, high heat transfer
rate boilers that can tolerate almost no solids internally. Boilers operating at pressures
at or below 800 psi (which is usually considered a borderline between industrial medium-pressure
and high-pressure boilers) may also employ high purity feedwaters. When a boiler's
feedwater routinely meets such high purity standards, the internal chemical program
of choice is normally a pH/phosphate control program (sometimes known as a coordinated
or congruent phosphate control program), optionally together with a dispersant treatment.
A pH/phosphate program differs from a standard or residual phosphate program in that
the phosphate is primarily added to provide a controlled pH range in the boiler water
to provide a buffering counteraction against acid and caustic attack of the internal
metallic surfaces, which is described in more detail below. A pH/phosphate control
program is very difficult and time consuming to control. The conventional control
of such a program is normally based on many assumptions about feedwater phosphate
demand, actual boiler cycles and boiler feedwater sodium levels. Feedwater phosphate
demand, as that terminology is used herein and generally understood in the boiler
field, is phosphate that becomes insoluble within a boiler system, for instance upon
the formation of insoluble phosphate species upon combination with calcium or iron
ions, or which hides-out (discussed elsewhere) during system load transients. Feedwater
phosphate demand is normally a fairly steady value in a well controlled boiler system.
[0006] It is an object of the present invention to provide a pH/phosphate control method
that is both cost efficient and relatively simple to operate. It is an object of the
present invention to provide a pH/phosphate program control process of great sensitivity
that is independent of fluctuations in a boiler's steaming rate. It is an object of
the present invention to provide a pH/phosphate program control method of great sensitivity
that is independent of small impurity variations in the boiler feedwater. It is an
object of the present invention to provide such a pH/phosphate program control method
for boilers having an operating pressure of at least 15 psi. These and other objects
of the present invention are described further below.
Disclosure of the Invention
[0007] The present invention provides a method of controlling a pH/phosphate program in
a boiler water system wherein steam is generated from the water within the boiler,
feedwater of high purity is fed to the boiler to replenish the water which leaves
the boiler as steam and blowdown, and the concentration of impurities in the boiler
water is reduced by withdrawing fractions thereof as blowdown. Such a boiler has a
concentration cycle value (boiler cycles), which is the value of the average concentration
("C
F") of an inert component in the blowdown at steady state divided by its concentration
("C
I") in the feedwater. Under a pH/phosphate treatment program, phosphate is fed to the
boiler in amounts to provide a sodium to phosphate ratio in the boiler water within
a target ratio range. The method of the present invention controls a pH/phosphate
program by employing a plurality of monitorings to obtain crucial data from which
the feedwater phosphate concentration, actual boiler cycles and boiler feedwater sodium
levels are determined, and integrating the information therefrom. The present invention
provides a means of addressing these three parameters (feedwater phosphate concentration,
actual boiler cycles and boiler feedwater sodium levels), which are root causes of
system variation. The present invention thereby makes it easier to stay within reasonable
control parameters with a pH/phosphate program and enhances water treatment chemical
control. This leads to a more effective pH/phosphate program and thus higher boiler
reliability and lower maintenance costs.
[0008] This invention uses:
(a) an inert tracer to determine boiler cycles and to set a correct phosphate feed
target as well as to control blowdown rate;
(b) an inert tracer to measure phosphate feed rate, which together with the boiler
cycles determination simplify the determination of the residual phosphate level in
the boiler; and in some embodiments also
(c) feedwater sodium analysis, which together with the alkalinity feed control (via
the introduction of an OH⁻-alkalinity source, preferably NaOH, with the feedwater)
permit a constant hydrate alkalinity level to be maintained in the boiler feedwater,
instead of having a varying feed rate that is a fluctuating value based on pretreatment
control as an input to the pH/phosphate control box.
[0009] The inert tracer overcomes problems with phosphate "hideout", which causes operators
to make incorrect program adjustments in their efforts to stay within their control
parameter range ("control box"), which is discussed in more detail below.
Brief Description of the Drawings
[0010] FIGURE 1 is a boiler system having monitoring and control means of the present invention
shown in diagrammatic form.
[0011] FIGURE 2 is a graphical record involving of continuous blowdown tracer monitoring
and boiler cycles during time period units T-1 through T-6.
[0012] FIGURE 3 is a pH/phosphate control curve.
Preferred Embodiments of the Invention
[0013] The preferred embodiments of the present invention include preferred embodiments
concerning monitoring the feedwater phosphate concentration and determining actual
boiler cycles, monitoring the feedwater phosphate concentration, determining actual
boiler cycles and monitoring boiler feedwater alkalinity levels, and preferred embodiments
concerning the pH/phosphate program control system in its entirety and integration
of data.
Caustic-Gouging Corrosion and pH/Phosphate Programs
[0014] As noted above, when high purity standards govern feedwater quality, the internal
water treatment program of choice for control of scale deposition and corrosion within
the boiler system is most often a pH/phosphate program. Stringent feedwater standards
are commonly employed for high pressure boilers, but not limited thereto.
[0015] High pressure boilers are no longer restricted to utilities. The increasing cost
of energy is making cogeneration much more popular. The petrochemical, paper, and
chemical industries now commonly use 1200-1800 psig boilers to provide cogeneration
of both electricity and steam. Condensing and backpressure turbines are used to drive
generators, compressors, and the like, while supplying extraction or exhaust steam
for plant use. Waste-heat boilers receive heat from process streams for the production
of steam. These high pressure cogenerative and waste-heat boilers have feedwater quality
standards at least substantially comparable to that of high pressure utility boilers.
In many cases, the initial design of high pressure waste-heat boilers may require
even stricter feedwater standards than a utility boiler of higher pressure.
[0016] As boiler pressures and heat transfer rates have increased, water-side caustic corrosion
of boiler tubes (caustic gouging) has increasingly become a problem. Caustic gouging,
or ductile gouging, starts to occur when caustic is allowed to concentrate against
hot boiler metal surfaces, dissolving the normally protective magnetite. Normal boiler
water hydrate alkalinity levels are harmless to boiler steel, but localized concentrations
of tens of thousands of ppm are very aggressive. At such points, the protective magnetite
on the water-side surface of the boiler tube wall is dissolved, as shown in the following
Equation 1.

Where the protective magnetite film is dissolved, the parent tube metal is exposed
and is susceptible to corrosion, as shown in the following Equations 2 and 3.
Equation 2 3 Fe + 4 H₂O → Fe₃O₄ + 4 H₂
Equation 3 Fe + 2 NaOH → Na₂FeO₂ + H₂
The concentration of boiler water chemicals at boiler surfaces, and the ensuing
corrosion, are the results of two prime mechanisms, i.e., deposit formation and film
boiling, which are discussed below.
[0017] Deposit formation on boiler surfaces (dirty boiler tube surfaces) is the most common
cause of localized concentration of chemicals to corrosive levels. The most prevalent
deposits that can cause surface-concentration of boiler chemicals are derived from
iron and copper corrosion products which enter a boiler with its feedwater. Large
industrial condensate return systems or extensive feedwater heating systems (as found
in a utility operation) are major sources of both iron and copper impurities which
can enter the boiler. These and other contaminants may enter a boiler in soluble form,
and then precipitate in the vicinity of the hot boiler surfaces. The higher temperatures
at a boiler's heat-exchange surfaces will precipitate contaminants whose water-solubilities
decrease at higher temperatures. The precipitation of contaminants leads to deposition
on the boiler surfaces. Iron compounds form porous, insulating-types of deposits that
are particularly active in promoting surface-concentration of boiler chemicals. Porous,
insulating-types of deposits allow boiler water to diffuse into the deposit where
the water becomes trapped and boils. The boiling of deposit-entrapped water produces
relatively pure steam which tends to diffuse out of the deposit, leaving behind superheated,
non-boiling equilibrium solutions of caustic. Boiler water containing, for instance,
100 ppm of NaOH can form solutions having from about 50,000 to about 400,000 ppm NaOH
(5 - 40%) upon diffusion into porous, insulating types of surface deposits.
[0018] Hydrogen cracking (embrittlement) of boiler steel can occur as an additional consequence
of high temperature zone deposit accumulations, (normally found only above 1800 psig).
This kind of boiler tube deterioration may accompany caustic gouging. In hydrogen
cracking, atomic hydrogen formed as a result of corrosion of the tube surface (from
alkali or acid attack) migrates or diffuses into the tube metal where it combines
with the carbon contained in the cementite (FeC) to form methane gas, as shown in
Equation 4.

Discontinuous, intergranular cracks are formed along the grain boundaries due to gas
pressure buildup.
[0019] Film boiling, the second primary cause of caustic gouging, occurs when the heat input
(heat flux) to a given section of boiler tube surface is so high that an orderly transfer
of the heat from the tube surface to the steam-water mixture does not take place,
leading to the formation of highly concentrated, nonboiling liquid films. Film boiling
(which is also called "departure from nucleate boiling" or "DNB", steam blanketing,
or steam disengagement) in most instances arises because the affected surface was
not intended to receive direct heat input, or the surface orientation (sloped, horizontal,
and so forth) is such that inadequate free rinsing occurs even though the heat inputs
experienced are normal. Insufficient water flow in a tube due to design or operational
considerations may also cause film boiling. Film boiling on the water-side metal surface
causes an evaporative concentration of salts.
[0020] Film boiling and the problems associated therewith are generally seen with increasing
frequency when:
(a) boiler pressures are increased;
(b) design heat transfer rates exceed 150,000 Btu/hr/sq ft.;
(c) boiler water circulation is disturbed due to weld backing rings, tube dents, or
unusual tube or boiler designs; and/or
(d) fireside abnormalities occur, such as unusual slagging (or deslagging) problems,
or physical damage or destruction to baffles, and so forth.
[0021] The elimination or reduction of surface concentration of caustic and the resulting
corrosion requires a boiler water treatment program that minimizes or excludes free
hydroxide alkalinity (caustic). Low-alkalinity boiler water treatment programs exemplified
by the pH/phosphate approach have become necessary. Two phosphate-based programs of
the pH/phosphate type provide low-alkalinity boiler water treatments. The "TRI-AD"
(or "Precision Control") program favors the maintenance of a low, tightly controlled
excess of caustic. The "Congruent Control" program operates well within a "captive
alkalinity zone" (described below) and essentially eliminates the potential for any
free caustic. While any program that advocates alkalinity (hydrate) control by using
various phosphate or phosphate-caustic ratios to maintain a safe boiler water environment
may be classified being of the pH/phosphate type, only these two specific variations
(TRI-AD and Congruent Control) will be discussed here in detail. Other pH/phosphate
control programs and other pH/phosphate programs are within the scope of the present
application, that is, they can also be controlled by the present method.
[0022] The intricate phosphate chemistry that controls these two programs requires very
pure feedwater, with make-up water of demineralized or evaporated quality. Even small
amounts of contamination can make control of a pH/phosphate program extremely difficult.
Return condensate should be very high quality, preferably having been polished through
powdered resin or deep bed ion exchange units. Table 1 below sets forth the feedwater,
boiler water, and steam quality control guidelines for these programs.
Table 1
Typical pH/Phosphate Operating Control Guidelines |
Water System/Parameter |
Operating Range or Maximum (max.) |
Returned Condensate/Feedwater |
|
Specific Conductivity |
< 5 micro-mhos |
Cationic Conductivity |
< 10 micro-mhos |
pH |
8.8 - 9.6 (see below) |
Na⁺ |
< 5 ppb |
O₂ |
< 7 ppb |
Turbidity |
< 10 ppb |
SiO₂ |
< 5 ppb |
Fe |
10 ppb max. |
Cu |
10 ppb max. |
Steam Generator |
|
Specific Conductivity |
< 2000 |
pH |
9.1 - 10.2 |
PO₄ |
2 - 25 ppm |
SiO₂ |
(limit varies with pressure) |
Saturated Steam |
|
Cation Conductivity |
0.1 micro-mhos max. |
Na |
3 - 10 ppb max. |
SiO₂ |
10 ppb max. |
[0023] Boiler systems containing copper alloys should have a condensate/feedwater pH within
the range of 8.8 - 9.2. All-steel boiler systems are best protected when the condensate/feedwater
pH is within the 9.2 - 9.6 range. A compromise condensate/feedwater pH control range
of 8.8 to 9.2 is usually established where both metals are present.
[0024] The basis of phosphate-pH control is that sodium phosphates are pH buffers and the
disodium phosphate specie transforms a potentially corrosive caustic-rich solution
into a relatively harmless trisodium phosphate solution, as shown by the following
Equation 5.

While the reaction shown in Equation 5 describes the basic concept phosphate-pH control,
the underlying chemistry is more complex. Many phosphates can be used in various combinations
to achieve the desired result. As shown below in Equation 6, the mono-, di- and trisodium
phosphates (orthophosphates) are all derived from the reaction of phosphoric acid
with caustic.
Equation 6 H₃ PO₄ + x NaOH ↔ Na
xH
3-xPO₄ + H₂O
wherein x is 1, 2 or 3. The addition of phosphoric acid or any one of the orthophosphates
to water produces a hydrolysis reaction that yields phosphate ions and hydrogen ions,
sodium ions, hydroxide ions or combinations, while caustic hydrolyzes to sodium and
hydroxide ions, as shown in Equations 7 to 11 of Table 2 below.

Only the dibasic hydrogen phosphate ion, HPO₄⁻², is shown in Equations 7-10, because
within a pH range of from about 9 to about 10.5 the distribution of the different
ionic species of phosphate is at most 1 or 2 percent of the mono- or tribasic phosphate
ions, and thus substantially all phosphates that are added to water within that pH
range will hydrolyze to the dibasic ion.
[0025] From Equation 5 taken together with Equations 8 to 10, the following conclusions
may be derived concerning the addition of sodium orthophosphates within the boiler
pH range:
(1) Tribasic sodium phosphate hydrolysis releases sodium hydroxide (Equation 10).
Thus, one mole of trisodium phosphate will increase boiler water pH equivalent to
the addition of one mole sodium hydroxide.
(2) Dibasic sodium phosphate hydrolysis has little or no effect on pH (Equation 9).
(3) Monobasic sodium phosphate dissociates to the dibasic form; thus, a one mole addition
of monosodium phosphate is capable of neutralizing one mole of sodium hydroxide or
one mole of trisodium phosphate.
[0026] The solution (boiler water) pH that will result from the addition of the various
orthophosphates therefore can be predicted, and graphs showing the phosphate-pH relationship
in terms of phosphate concentration as PO₄ (in ppm) versus pH of an aqueous solution
for various Na:PO₄ ratios are available in boiler literature. (Since orthophosphates
are comprised of sodium and phosphate in molar ratios of 1, 2 and 3, it is common
practice to describe an orthophosphate or a mixture of orthophosphates in terms of
its sodium-to-phosphate molar ratio, Na:PO₄).
[0027] Sodium hydroxide generated solely by the hydrolysis of trisodium phosphate (Equation
10) is sometimes called "captive" because it will revert to trisodium phosphate at
any site of localized evaporative concentration. Such reversion of sodium hydroxide
to trisodium phosphate in confined areas such as within and/or under surface deposits
avoids the formation of pockets of concentrated sodium hydroxide and thus prevents
caustic-gouging type of metal attack. In Equation 10, when water is removed upon evaporation,
the equilibrium is forced to the left. A complete evaporation to dryness would leave
a residue of trisodium phosphate free of sodium hydroxide, but incomplete evaporation
is the more likely condition beneath a porous deposit. Incomplete evaporation produces
a liquid underneath the deposit that is rich in sodium hydroxide, particularly if
incipient localized corrosion is already occurring. Thus the maintenance of a 3:1
sodium phosphate ratio in boiler water may not provide positive protection against
caustic-concentration-type corrosion damage. One form of pH/phosphate control maintains
a Na:PO₄ ratio that does not exceed 2.6. (A Na:PO₄ ratio of 2.6 corresponds to a 3:2
blend of trisodium and disodium phosphate.) The differing hydrolysis effects of different
sodium phosphates when selectively adjusting pH, PO₄, or both, to keep pH and PO₄
coordinates within the desired range are shown in a control diagram discussed below.
[0028] The primary objective of a pH/phosphate program, whether of the TRI-AD or Congruent
Control type, is controlling the presence or absence of free sodium hydroxide. At
a 3.0:1 Na:PO₄ mole ratio or greater, only the equivalent of trisodium phosphate plus
caustic exists. At Na:PO₄ mole ratios below 3.0:1 (down to a 2.0:1 mole ratio), a
mixture of disodium and trisodium phosphate is present. In Figure 3 is shown a pH/phosphate
control curve which includes the equilibrium boundary ("ratio 3.0" line) between the
trisodium phosphate/caustic combination and the trisodium/disodium combination for
pH values from about 8.2 to about 10.6, which forms the basis for all phosphate-pH
programs. The line represents the points (combinations of pH values versus phosphate
concentrations) at which there exists a 3.0:1 Na:PO₄ mole ratio and only the trisodium
phosphate specie is present (the phosphate-pH relationship for trisodium phosphate
or 3.0:1 Na:PO₄ line). Above that boundary, the equivalent of trisodium phosphate
and caustic exist. Below that boundary, a mixture of tri- and disodium phosphate exists.
[0029] The TRI-AD pH/phosphate program operates above the 3.0:1 Na:PO₄ line (Figure 3);
the Congruent Control pH/phosphate program operates below the 3.0:1 line, which is
referred to in the technical field as the "captive alkalinity" zone.
[0030] As boiler water of a given Na:PO₄ ratio is concentrated at local areas of the boiler
(under deposits or in zones of film boiling), the sparingly soluble natures of the
various phosphate species at boiler water pH values and temperatures cause precipitation
(solid phase formation) to occur. The solid phase Na:PO₄ ratio will always be less
than 3.0:1, but the actual solid phase stoichiometry will vary with the existing liquid
phase Na:PO₄ ratio. When the liquid phase Na:PO₄ ratio is greater than 2.85:1, the
solid phase formed upon precipitation will have a lower Na:PO₄ ratio than the liquid
phase. Phosphate precipitation when the liquid phase Na:PO₄ ratio is greater than
2.85:1 therefore results in an increase in the liquid phase Na:PO₄ ratio and causes
a shift towards the 3.0:1 Na:PO₄ ratio line, above which free sodium hydroxide will
exist, creating potential corrosion problems. Conversely, if the liquid phase Na:PO₄
ratio is less than 2.85:1, the solid phase Na:PO4 ratio is always greater than the
liquid phase ratio. Phosphate precipitation when the liquid phase Na:PO₄ ratio is
less than 2.85:1 therefore depletes the water system of sodium, decreasing the Na:PO₄
ratio of the remaining liquid phase. Precipitation thereby moves the water system
even further below the 3:1 line, rather than toward it.
[0031] A plot of the composition of solid in terms of the Na:PO₄ ratio versus the composition
of solution phase solid in terms of the Na:PO₄ ratio in an equilibrium system of disodium
phosphate/water at a given temperature will cross a congruence line (line formed of
points representing equal solid and solution Na:PO₄ ratios) at what is known as a
point of congruency, or simply the congruent point. For instance, at a temperature
of 572 °F (300 °C) the congruent point is where both the solution phase and the solid
phase have an identical Na:PO₄ of 2.85:1. At 689 ⁰F (365 °C), the congruent point
is about 2.6:1. Since these congruent points form a line of demarcation between the
zones where you are either moving towards or away from the 3:1 line under conditions
of precipitation, these points become the upper control boundary for the Congruent
Control method. Typically under Congruent Control the phosphate-pH relationship is
controlled to maintain a solution phase (boiler water) Na:PO₄ ratio range of 2.6:1
to 2.3:1. The 2.3:1 ratio is chosen as the bottom of the control range to avoid the
formation of acid phosphate which can occur below 2.13:1.
[0032] "Phosphate hideout" and "chemical hideout" are terms used to describe the loss of
boiler water phosphate residual that accompanies this solid phase formation problem.
Typically, phosphate tends to "disappear" as load (steaming rate load) is increased
towards maximum. The pH will also vary, usually in an upward direction, although one
would expect that the change in pH would be consistent with the change in Na:PO₄ ratio
of the solution. The upward trend in pH may be explained by an interaction between
phosphate and magnetite that is also known to exist. Just as phosphate disappears
with increasing load, so does it reappear with decreasing load, with concurrent impact
on the system pH. Dissolution of the solid phase occurs as load is reduced.
[0033] The phosphate hideout phenomenon can pose a significant chemistry control problem
in high pressure boilers that experience load variations. In some severe instances
"all-volatile treatment" (AVT) programs, for instance using only nitrogen-hydrogen
compounds such as ammonia and hydrazine, have been adopted to avoid the control problems
caused by phosphate hideout. AVT programs, however, are not universally appropriate.
In an All Volatile Treatment program the boiler water is unbuffered and thus it is
subject to extensive and rapid pH excursions in the event of feed water contamination,
and therefore such a program has no tolerance for condenser coolant inleakage unless
condensate polishing is provided. An AVT program Is therefore a realistic alternative
only when feedwater and condensate quality meet the rigorous standards required for
AVT.
[0034] In the 1000-1800 psi (70,307-126,553 grams/sq. cm) range (large industrial boilers
and small utility boilers), a TRI-AD program that incorporates the maintenance of
very low levels of free hydroxide alkalinity have at times been preferred over the
more stringent Congruent Control approach. The TRI-AD program control, however, is
not less precise, and the control principles are not any less complicated, than a
Congruent Control program. (In fact, as shown in Figure 3 discussed further below,
the boiler water total solids level on a TRI-AD program may actually be lower than
on Congruent Control program.) A TRI-AD program's primary advantage in systems where
it has been shown to be acceptable is that it provides a small reservoir of excess
caustic that can:
(a) help to condition any hardness sludge that may form as the result of feedwater
contamination; and
(b) buffer against unusual downward excursions in pH, which also typically are a result
of feedwater contamination, particularly when the feedwater contamination is due to
a condenser leak.
A TRI-AD program cannot be used when operating experience indicates a system intolerance
to free caustic, nor when the pressure is above 1800 psi (126,553 grams/sq. cm).
[0035] In Figure 3 are shown the control parameters for coordinated programs of the Congruent
Control and TRI-AD types. Figure 3 is comprised of plots of the total orthophosphate
in the boiler water, in terms of ppm PO₄ versus boiler water pH at 25 °C (77 °F).
Figure 3 also shows the specific phosphate - pH coordinate boundaries based on boiler
drum pressure. Shown is a Box A (circumscribing Area A) and a Box B, Box B encompassing
the combined Area A and Area B. Also shown in Figure 3 is Box C (circumscribing Area
C) and Box D, Box D encompassing the combined Area C and Area D. Both of Boxes C and
D are bounded by the 2.3 and 2.6 Na:PO₄ ratio lines. Using a TRI-AD program (Box A
and Box B), a boiler operating in the 1000-1500 psi range could use PO₄ - pH coordinates
over the combined A and B areas, whereas a boiler operating in the more critical 1500-1800
psi range is restricted to Area A. The Congruent Control program, which as mentioned
above is a pH/phosphate - pH program that takes its name from the concept of a point
of congruency existing between solid phase and liquid phase Na:PO₄ ratio, seeks to
minimize, if not eliminate, the possibility of developing free caustic anywhere in
the boiler. It operates in Boxes C and D, between the 2.6 and 2.3 Na:PO₄ ratio lines.
Any phosphate hideout (
e.g., precipitation due to localized areas of evaporation, hence potential corrosion)
results in a swing in the Na:PO₄ ratio away from the 3:1 line, and thus away from
potential "free" hydroxide alkalinity.
[0036] Most modern utility boilers use the Congruent Control mode of the pH/phosphate-pH
concept. A complete program routinely further includes hydrazine or another oxygen
scavenger, and a neutralizing amine or ammonia for feedwater and condensate pH control.
[0037] The control vectors on Figure 3 provide the direction in correcting a transient (fugitive)
condition in control. A line drawn between the current, or existing, PO₄/pH coordinates
versus the desired PO₄/pH coordinates identifies the appropriate directional vector
that indicates which phosphate (or phosphates) need to be added, or whether caustic
addition or boiler blowdown may be more appropriate. These vectors are founded on
the basic orthophosphates hydrolysis reactions discussed above.
[0038] In either the TRI-AD or the Congruent Control program, actual choice of phosphate
product(s) (phosphate feed) used to control the system may vary with plant or company
preference. Liquid phosphate products are considered by some plants the most convenient
form and these are commercially available, for instance under the trade names of Nalco
7208 and 7209 from Nalco Chemical Company of Naperville, Illinois. Phosphate feeds
comprised of monosodium phosphate and caustic in combination may provide wide flexibility.
Mixtures of two or more phosphates, changing the ratio as needed to maintain control,
are used in some plants. In some cases, there may be sufficient "outside" sodium alkalinity
entering the system (most often as a result of sodium alkalinity inleakage) to allow
standardization on one phosphate feed source, but typically the outside sodium alkalinity
level varies significantly and thus instead the phosphate feed mixture requires constant
adjustment when outside alkalinity is entering the boiler.
[0039] The conventional servicing and testing of high pressure boiler systems is conceptually
no different than for lower pressure boiler systems, and most of the same test procedures
are used, although the lower concentration levels of a conventional pH/phosphate program
control require exemplary analytical technique. In most plants, control of boiler
water treatment will be under the direction of a chemist. A water treatment laboratory
may be only minimally equipped or may have an array of sophisticated equipment (atomic
adsorption, specific ion electrodes, spectrophotometers, and the like). In addition
to laboratory analysis capability, most plants will also have several continuous reading
automatic analyzers throughout the plant to support the maintenance of the stringent
water quality requirements of pH/phosphate programs.
Phosphate Concentration
[0040] The present invention includes the determination of the phosphate concentration in
the feedwater and optionally determining whether there is a difference between the
actual feedwater concentration of the phosphate and the target feedwater concentration
and correcting the dosage of the phosphate if a difference is detected. In these steps,
at least one phosphate is added to the fresh feedwater prior to charging it to the
boiler to provide a target concentration of the phosphate in the boiler water. The
additional steps are:
1. Adding a tracer to the fresh feedwater in a dosage of known proportion to the phosphate
and/or phosphate-containing treatment product being added to the feedwater;
2. Sensing a characteristic of the tracer in the feedwater downstream of its addition
point that is equivalent to its concentration in the feedwater;
3. Converting the sensed characteristic to a value equivalent to the concentration
of the inert tracer in the boiler water;
4. Determining from the value equivalent to the concentration of the tracer a value
equivalent to the concentration of the phosphate in the feedwater, and optionally
whether a difference between the target concentration and the actual concentration
of phosphate exists; and
5. Correcting the dosage of phosphate if the determination shows the existence of
such a difference.
[0041] For the determination of phosphate concentration, the tracer may be inert or active,
as discussed below, and the tracer characteristic sensing is conducted at a point
that is preferably downstream of the tracer and phosphate addition point(s) and upstream
of point that the feedwater enters the boiler compartment.
Boiler Cycles
[0042] A boiler's cycles of concentration is an operating parameter that should be well
monitored. Limits on boiler cycles of concentration, so as to limit the maximum impurity
concentration within a boiler, are routinely set by boiler and turbine manufacturers,
water treatment companies and the industrial plants employing the boilers. Such limitations
are intended for, and are generally necessary to, the avoidance of serious scale formation/deposition
on internal boiler surfaces and/or boiler-related equipment surfaces within a boiler,
despite an otherwise adequate water treatment program. Boiler feedwater, which normally
is comprised of both makeup water and recirculated condensate water, contains some
impurities regardless of the extent to which such waters are treated before being
fed to a boiler. When steam is generated, substantially pure H₂O vapor is discharged
from the boiler, leaving the impurities (the dissolved and suspended solids) behind,
which increases their concentration in the boiler water. The discharged steam is replaced
by contaminant-containing feedwater. An ever increasing concentration of dissolved
and suspended solids in the boiler water would inevitably result in very serious problems,
including deposit formation, corrosion, foaming, mechanical and selective carryover,
decreased heat transfer efficiency, boiler tube failure or occlusion, and the like.
Boiler-impurities concentration (boiler solids concentration) is offset by withdrawing
water as blowdown. The heat energy in the blowdown, however, is a major factor reducing
a boiler's thermal efficiency, and therefore a blowdown rate in excess of that required
to limit the solids concentration is preferably avoided. An excessive blowdown rate
also unnecessarily increases water costs.
[0043] The ratio of solids concentration in the blowdown to the solids concentration in
the feedwater is called the "cycle value", "cycles of concentration", "concentration
cycles" or "cycles" of a boiler operation. If a boiler is being operated on too low
of a concentration cycle, the blowdown rate is too high, heat energy is lost and the
boiler operation is unduly inefficient. The needless loss of heat with unrequired
blowdown decreases thermal efficiency, and water costs increase at the same time.
If a boiler is being operated on too high of a concentration cycle, the blowdown rate
is too low and the potential for solids-derived problems, such as scale deposition
and corrosion (discussed above), are increased. Thus the cycles of concentration is
one of the primary operating parameters of a boiler.
[0044] The method of the present invention includes the monitoring of a boiler operation's
cycles of concentration. U.S. Patent No. 5,041,386, Claudia C. Pierce, Roger W. Fowee,
and John E. Hoots, issued August 20, 1991, incorporated hereinto by reference, discloses
the use of inert tracers to monitor boiler concentration cycles. Boiler cycles are
calculated by adding an inert tracer to the feedwater being charged to the boiler
in a known concentration, and then determining an analog of its concentration in the
blowdown. If the cycles value does not compare to the standard operating value, which
may be the cycles value proposed by the boiler manufacturer or set by the operator
or suggested by the water-treatment supplier, then the blowdown rate and/or the dosage
of water-treatment agent can be adjusted. The inert tracer preferably is employed
to determine the cycles (impurity or contaminant concentration within the boiler water)
on a continuous basis. The inert tracer may be used for other purposes, such as the
determination of percent holding time (half-life time), as a reference standard in
the monitoring of concentration of water-treatment agent concentration level, and
the like, concomitantly with the cycles determination.
[0045] Industrial boilers often have heat transfer rates in excess of 100,000 Btu/ft²-hr
(2,500 cal/m²-hr) and the presence of even an extremely thin deposit layer within
the boiler could cause a serious elevation in the temperature of the tube metal. Therefore,
as discussed above, the required feedwater purity is very high and the permitted concentration
of impurities introduced with the feedwater is very low. These are often high cycles
value boilers with almost constant steam generation demands.
[0046] Boiler cycles (cycles value, concentration cycles, cycles of concentration and the
like) as such terminology is used herein, as understood generally in the boiler field,
is the ratio of the average concentration of a particular impurity or component ("IMP")
in the blowdown at steady state (average final concentration or "C
F") to its concentration in the feedwater (initial concentration or "C
I"), which ratio is determined from the following Formula I:

and the value, which is an equilibrium value, will always be greater than one since
the impurity IMP in the blowdown is always more concentrated than in the feedwater
due to water removal as steam. (Upsets in feedwater quality of a magnitude sufficient
to lower the cycles value to less than one are not tolerable in the boiler field and
would never occur outside of a major boiler operational failure.)
[0047] In the process of the present invention, the inert tracer is the "impurity" or "IMP"
of Formula I above, and the acronym "IT" for "inert tracer" can be employed in substitution
for the "IMP" designation. When the cycles values of intermediate and high pressure
boilers are monitored with inert tracers (for instance those disclosed in U.S. Patent
No. 5,041,386) that do not appreciably carry over into the steam, and which can be
selectively detected at very low concentrations (for instance 0.005 ppm or less),
it has been found that in a given boiler system not only can the average cycles value
be determined from the average C
F, but the normal range of the C
F fluctuation can also be determined.
[0048] The concentration of the inert component in the boiler at steady state varies from
a high concentration "C
H", having a value that is higher than C
F (the value of the average concentration of an inert component in the blowdown), to
a low concentration "C
L", having a value between the (C
I) and the (C
F), within a time period A as shown graphically in Figure 2. The high concentration
C
H of the inert tracer in a boiler for a uniformly cycling boiler system is of course
higher than the average C
F of the blowdown. The low concentration C
L of the inert tracer in the boiler is lower than the average C
F of the blowdown but, since the boiler water is never entirely replaced with feedwater
during cycling, it is never as low as C
I.
[0049] The present invention contemplates the phenomenon that in any practical boiler system
operation the C
F is not an absolute constant and the cycles value assigned to a boiler operation is
based on an average value of C
F. The monitoring of a boiler cycles value with an inert tracer can also be done for
boilers that are not intermediate or high pressure boilers. (The only boiler operation
parameters that may exclude the use of some of the preferred embodiments of present
invention are pressures in excess of 1,800 psig, which pressures may lead to decomposition
of the presently preferred sulfonated naphthalene inert tracers that are described
below.)
[0050] Steady states as such terminology is used herein and as understood generally in the
boiler field, is the condition that exists when the concentration of the inert tracer
(or other stable substance introduced into the boiler with the feedwater) in the water
of the boiler system reaches a uniform, repetitious cycling fluctuation of the inert
tracer from a predictable high concentration (C
H) to a predictable low concentration (C
L), which state is reached when the transient conditions arising at the start of the
inert tracer feed to the feedwater have become negligible. For purposes of the present
invention, a boiler can be considered operating at a steady state with respect to
the inert tracer when there is no significant change in the fluctuation of the inert
tracer between C
H and C
L of the concentration of the inert tracer in the boiler and no significant change
in the blowdown rate, the feedwater rate, the rate of feeding the tracer to the boiler
(or concentration of tracer in the feedwater when introduced therewith) and steaming
rate (in the absence of boiler leakage). To detect a boiler's cycles value employing
the present process, a steady state as to inert tracer concentration must first be
reached, and preferably the C
L and/or C
H values or values proportionate thereto, determined.
[0051] As mentioned above, the cycles-determination step of the present process can be activated
only after a steady state as to the inert tracer has been reached. The time required
for reaching such steady state after a uniform dosage (feed rate) of inert tracer
has begun can be calculated from the following Formula II:
wherein M is the mass of the boiler water (in lb.), B is the blowdown rate (in lbs/hour),
C
F is the concentration or average concentration of the inert tracer in the blowdown
after steady state is reached, and C
T is the concentration of the inert tracer in the blowdown at time t. The M/B factor
is a constant (boiler constant "K") for a given boiler operating under a uniform blowdown
rate. M can be determined from design documents or by spiking the boiler quickly with
a known amount of an inert tracer and measuring the value C
F obtained with the blowdown shut off. If the mass of the boiler water is unable to
be known precisely, a determination that a steady state as to the inert tracer has
been reached is made when a uniform average cycles value is seen by monitoring the
inert tracer for a period of about one to two weeks while holding C
I constant.
[0052] The cycles determination of the present method is comprised of the following steps:
1. Employing as the inert component an inert tracer added to the feedwater in a known
concentration (CI);
2. Sensing a characteristic of the inert tracer in the boiler at steady state equivalent
to its concentration in the boiler water;
3. Converting the sensed characteristic to a value equivalent to the concentration
of the inert tracer in the boiler water;
4. Optionally recording the cyclic concentration fluctuation from CH to CL of the inert tracer in the boiler water;
5. Determining the cycles value of the boiler from the equation

6. Optionally controlling the blowdown rate based on the above determination of the
cycles value.
[0053] The present invention may be employed regardless of a boiler's normal blowdown flow
operation and control. In many large boilers the blowdown valve is normally open and
the blowdown rate would not normally fall to zero. Instead the blowdown valve adjusts
the blowdown rate depending upon some indicated impurity build-up within the boiler,
which indication may be based on concentration readings of an inert tracer in the
boiler as it fluctuates between C
H and C
L. A balance between the rate feedwater is flowing into the boiler and the rate steam
is being discharged is generally seen in this type of boiler.
[0054] A pH/phosphate program can also be controlled in a boiler with open/close blowdown
valve operations which has a valve frequency time period B measured from the closing
of the blowdown valve, shutting off the blowdown flow, to the time at which the blowdown
valve would normally reopen. In such a boiler the time period B would be about equivalent
in length to time period A.
[0055] In preferred embodiments of the present invention, the cycles value determination
is performed by monitoring an inert tracer in both the feedwater and in blowdown from
the boiler and the phosphate feedwater concentration determination is performed by
monitoring an active or inert tracer in the feedwater. At the time of its discharge,
any blowdown stream will have the same composition as the water retained within the
boiler system at that time. The concentration of any substance in blowdown, therefore,
is commensurate with the concentration of that substance in the boiler at the time
the blowdown is discharged from the boiler. Blowdown, as such terminology is understood
generally in the boiler field, is water discharged in some manner from a boiler system.
A sidestream that continuously taps the boiler water for a continuous monitoring of
tracer concentration within the boiler would be considered a continuous blowdown stream
under this general definition. The concentration of the inert tracer in the boiler
at any point in time is preferably determined from the concentration of the tracer
in a sidestream that taps the boiler water, which preferably is a sidestream off the
blowdown line and which preferably taps the boiler water on a continuous basis. The
stream of water from a boiler that is monitored for purposes of the present invention
may be, for instance, a sidestream off a constantly-flowing blowdown stream. Such
monitored boiler-water stream also may be a stream from a separate boiler outlet,
or a sidestream from the blowdown line ahead of the blowdown valve in a boiler having
an intermittent blowdown flow. In any of such instances the monitored stream is not
only a part of the blowdown, but also of a generally known rate.
[0056] The inert tracer selection(s) for the purposes of the present invention must consider
the temperature constraints existing on the waterside of a boiler. The inert tracer
preferably is a fluorescence tracer, as discussed below.
Determination of Boiler Cycles
[0057] In FIGURE 1 there is shown in diagrammatic form a boiler system designated by the
general reference numeral 10, comprised of a boiler 12, a feedwater line 14 through
which feedwater flows into the boiler 12, a steam line 16 through which the steam
generated leaves the boiler 12, and a blowdown line 18 through which blowdown is discharged
from the boiler 12. The delivery of feedwater to the boiler 12 is controlled by a
feedwater pump 19. The discharge of blowdown from the boiler 12 is controlled by a
blowdown valve 20. A sidestream 28 off the blowdown line 18 supplies a small continuously
cooled sample stream for monitoring the concentration of the inert tracer in the boiler
by means of appropriate instrumentation 30, which is shown diagrammatically in FIGURE
1, and is discussed in more detail below. A feedwater sidestream 32 is tapped off
of the feedwater line 14 and along such feedwater sidestream 32 is a feedwater-monitoring
instrumentation 34, discussed below. The rate at which blowdown is discharged from
the boiler 12 is dictated by the balance desired between the rate of introduction
of impurities ("solids") to the boiler 12 together with feedwater and the rate of
solids discharge from the boiler 12 with blowdown. The desired balance is normally
an equal balance. The solids discharge rate should equal the solids introduction rate
over a given time period. The concentration of solids within the boiler 12 at any
given time, after steady state is reached, should also fall within predetermined limits.
[0058] This balance between the rate of introduction of impurities ("solids") to a boiler
together with feedwater and the rate of solids discharge from the boiler with blowdown
may be represented by a hypothetical boiler operation example as follows:
(1) The maximum solids concentration within the boiler at any given time is set for
an equivalent of 10 mg solids per liter of boiler water;
(2) The feedwater has an average solids concentration of 1 mg/liter, is fed to the
boiler at a rate of 1,000,000 lb/day and thus 1 lb. of solids are fed to the boiler
per day;
(3) The average solids concentration in the blowdown is equal to the 10 mg/liter solids
concentration maximum (see 1 above), 1 lb. of solids must be discharged per day (see
2 above) and thus the blowdown rate must be 100,000/lb per day; and
(4) Steam having an essentially zero solids content is generated and discharged from
the boiler at a rate of 900,000 lb. per day.
[0059] The desired cycles value for such a boiler balance is 100/10 = 10. That is, the concentration
of solids in the blowdown should be 10 times the concentration of solids in the feedwater.
However, the typical feedwater solids concentration, due to the pretreatment system
operation and the quantities of returned condensate, will have between a 5% and 25%
variance range. The more pure the feedwater, the greater the percentage variance range
and thus the more difficult it becomes to make accurate boiler cycle measurements
from normal feedwater impurities. The actual cycles value is more accurately measured
by continuously adding a uniform dosage of an inert tracer to the feedwater and monitoring
its concentration in both the feedwater and blowdown, along the feedwater line and
blowdown line respectively, and substituting the concentrations determined into Formula
I. (It is desirable but not required that the inert tracer be monitored along the
feedwater line for the cycles determination portion of the present invention, but
a tracer, which may or may not be inert, must be monitored along the feedwater line
for the phosphate concentration determination portion of the present invention. Moreover,
while using the same tracer for dual feedwater/blowdown cycles analyses is convenient,
separate tracers can of course be employed.) If the cycles value determined by the
inert tracer is lower than desired, the blowdown rate can be lowered, and if the cycles
value determined by the inert tracer is higher than desired, the blowdown rate can
be increased.
[0060] When the blowdown rate is set to provide the desired cycles value, the sidestream
28 off the blowdown line 18 (which need only comprise a negligible fraction of the
boiler water) is monitored to determine on a continuous basis the concentration of
the inert tracer(s) in the boiler water. The sidestream 28 is shown ahead of the blowdown
valve 20, which is a preferred position for the sidestream 28 regardless of the normal
blowdown valve operation. The concentration of the inert tracer added in known proportion
to the feedwater, as monitored by the instrumentation 30, will normally fluctuate
from the C
L low value to the high value C
H as described above. The instrumentation 34 may include one or more setpoints to support
the continuous monitoring of the boiler cycles. A low setpoint, for instance, may
be at or somewhat below the normal C
L value. A high setpoint may be at or somewhat below about C
H. For instance, if the inert tracer concentration in the feedwater of a boiler having
a cycles value of 50 is 1 ppm, its average concentration in blowdown (average C
F) would be 50 ppm. Its high value in the boiler C
H may be about 55 ppm, and its low value C
L may be about 45 ppm. If the tracer concentration in the boiler fell to 40 ppm, there
would be a 20 percent difference between this reading and the average C
F reading of 50 ppm, and a readily detectable difference of about 11 percent between
this reading and the normal low inert tracer reading C
L. There may also be a change in the feedwater rate if the boiler is a constant steam
load boiler, but this feedwater rate change would be extremely difficult to detect
with any assurance. A normal feedwater rate analyzer has an accuracy of about ± 3%
and a feedwater rate change would only be on the order of only about 0.5%. (In a constant
steam load boiler, the feedwater rate is the steam rate plus the blowdown rate, and
the steam rate for a boiler having a cycles value of about 50 would be about 50 times
the blowdown rate. Flow meters to determine blowdown flow rates often have accuracy
problems because of deposition and erosion.)
[0061] The downswing in the tracer concentration to its C
L value occurs when blowdown water leaves the boiler with a proportional amount of
inert tracer and the feedwater control system has replaced that blowdown with water
containing a lesser concentration of the inert tracer, diluting the overall tracer
concentration in the boiler water. The concentration of the inert tracer is not affected
by the feedwater purification efficiency, or other variable parameters of the feedwater.
The water loss and replacement, and the concomitant inert tracer dilution, is detectable
and measurable to a precise degree. For example, a change in blowdown rate that causes
only a 0.5% increase in water consumption by the boiler would also provide a 20% decrease
in the tracer concentration from its average of 50 ppm. The amount of the water consumption
and the precise dilution of the inert tracer need not be quantitatively determined,
although such quantitative determinations can be easily made using the method of the
present invention. The fluctuation of the inert tracer concentration in the blowdown
from C
H to C
L or vice versa during time period A can provide a graphical record of the boiler cycles
as shown in FIGURE 2.
[0062] The present invention of course goes farther than just the determination of the cycles
value. For purposes of the present invention the same inert tracer or a another tracer
is introduced into the boiler system in a known and uniform proportion to the phosphate,
and preferably, but not necessarily, it is introduced into the boiler system as a
component of the phosphate feed to the boiler or boiler feedwater in a known and constant
concentration proportion with respect to the phosphate. Since routinely the phosphate
feed rate will be in constant proportion to the feedwater feed rate when the cycles
value of the boiler is accurately determined and set at its optimal value, at least
over a time period of several days or a week or so, a separate (another) tracer to
measure the phosphate feed can, but need not, be used, and instead a single inert
tracer may be used for both the phosphate concentration and boiler cycles determinations.
The process of the present invention in an embodiment preferably includes a continuous
monitoring of the concentration of a single inert tracer in the feedwater and in the
boiler blowdown for both the phosphate concentration and boiler cycles determinations.
[0063] In the process of the present invention the boiler cycles and phosphate concentration
are determined, and a pH/phosphate program is controlled through an enhanced technique.
As shown in Figure 3 for the TRI-AD and Congruent Control types of pH/phosphate programs
(although the present invention is not limited to these types of pH/phosphate programs),
a line drawn between the existing PO₄/pH coordinates and the desired PO₄/pH coordinates
identifies the appropriate directional vector that indicates what parameter needs
to be increased to approach or regain the target coordinates. For instance, with reference
to Figure 3, if the line drawn is directed to about a 7 o'clock position, it substantially
coincides in direction with the blowdown control vector, and increasing the blowdown
rate will take the system towards the desired PO₄/pH balance (corresponding to the
desired coordinates on the graphs). If the line drawn is directed to about a 3 o'clock
position, it substantially coincides with the disodium phosphate control vector, and
increasing the disodium phosphate concentration (feed rate) will take the system towards
the desired PO₄/pH balance. If the line drawn is directed to about a 9 o'clock position,
it substantially falls between the blowdown control vector and the caustic control
vector, and increasing both the blowdown rate and the caustic concentration (by increasing
the caustic feed rate) in needed proportion to provide in combination the correct
vector as direction and length will take the system towards the desired PO₄/pH balance.
[0064] Referring again to Figure 1, the process of the present invention comprises:
(1) determining the phosphate concentration (feed rate) (which in preferred embodiments
is commensurate with the determination of the feedwater feed rate) with a tracer using
instrumentation 34 along the sidestream 32 off the feedwater line 14;
(2) determining the boiler cycles with a tracer (the same tracer or different tracers)
using at least instrumentation 30 along the sidestream 28 off the blowdown line 18;
(3) determining the blowdown pH, for instance with a continuously-reading pH meter
or other suitable instrumentation, shown in Figure 1 as instrumentation 36 downstream
of instrumentation 30 along the sidestream 28 off the blowdown line 18;
(4) optionally determining the hydrate alkalinity concentration of the feedwater,
for instance with an Orion 1000 series or other suitable sampling/analysis instrumentation,
shown in Figure 1 as instrumentation 38 downstream of instrumentation 34 along the
sidestream 33 off the feedwater line 14; and
(5) furnishing the data defining all of these value to a control center 40.
[0065] The control center 40, which preferably is a computer, provides a responsive control
of the caustic pump 44 (controlling caustic feed), the phosphate pump 46 (controlling
phosphate feed), and the blowdown valve 20 (controlling blowdown rate). The responsive
adjustments of the control center 40 thus are directed to the parameters subject to
potential adjustment to approach or regain the desired PO₄/pH balance (control box
PO₄/pH coordinates as shown in Figure 3, or control box PO₄/pH coordinates of like
PO₄/pH plots of pH/phosphate programs).
[0066] Referring to Figure 3 also, for any pH/phosphate program it may be desirable to maintain
a specific Na:PO₄ mole ratio ("R
m"), for instance a R
m within the range of 2.3 to 2.8 within the boiler. R
m is related to the Na:PO₄ concentration by molecular weight ratio ("R") by the factor
of 4.13 (that is, 95/23). The R
m values are shown on Figure 3 as lines, some of which provide the upper or lower boundaries
of the control boxes. While there is some system demand for phosphate in a pH/phosphate
program, it is not a high system demand, and more important than the actual unprecipitated
phosphate concentration value in the boiler 12 is that a uniform phosphate residual
level is maintained within the boiler 12. The two feedwater monitorings, that is the
feedwater analysis of the sodium concentration using sampling/analysis instrumentation
38 and the feedwater tracer (proportional to the phosphate being fed) using instrumentation
34 are sufficient to provide the control center 40 with values for the calculation
of R and/or R
m, whichever is more convenient. The pH monitoring of the blowdown using instrumentation
36 provides the control center 40 with the existing pH value within the boiler. The
determination of the actual boiler cycles through at least the monitoring of an inert
tracer at least in the blowdown using instrumentation 30 provides the control center
40 with the existing blowdown rate. Based upon this input of values, the control center
40 will produce the modifications in the caustic feed, the phosphate feed, and/or
the blowdown rate by appropriate adjustments to the caustic feed pump 44, the phosphate
feed pump 46, and/or the blowdown valve 20 that will create accurate overall program
vector adjustments within the control box.
[0067] The present invention provides a pH/phosphate control method that is cost efficient
and is relatively simple to operate; it reduces operator testing commitments, improves
boiler efficiency and reliability, and improves the process capability of the treatment
program. The present invention provides a pH/phosphate program control process of
great sensitivity that is independent of fluctuations in a boiler's steaming rate
or phosphate hideout tendencies. The present invention provides a pH/phosphate program
control method of great sensitivity that is independent of small quality variations
in boiler feedwater.
[0068] The present process also overcomes problems with phosphate "hideout", which causes
operators to make incorrect program adjustments in their efforts to stay within their
control parameter range. The phosphate concentration for purposes of pH/phosphate
program control in the process of the present invention is monitored in the feedwater.
The phosphate hideout seen as a loss of boiler water phosphate residual that accompanies
a solid phase formation is a control problem only when boiler-water phosphate residuals
are being measured by the phosphate concentration seen in the blowdown. The boiler-water
phosphate, as measured by blowdown phosphate concentration, disappears with increasing
load, and reappear with decreasing load, with concurrent impact on the system pH.
The process of the present invention is independent of blowdown phosphate concentration
measurements, and hence immune to erroneous control adjustment triggered by the phosphate
hideout phenomenon. The process of the present invention is thus particularly advantageous
for use in high pressure boilers that experience load variations, which otherwise
may adopt an AVT program to avoid the control problems caused by phosphate hideout,
or which are plagued by phosphate hideout control problems because their feedwater
and condensate quality do not meet the rigorous standards required for AVT.
Inert And Active Tracers
[0069] The terms "tracer" or "tracer(s), as used herein, refer to any and all of the tracers
used in the present process, regardless of whether a given tracer is active or inert
and regardless of whether a given tracer is a "feedwater tracer", a "phosphate-feed
tracer" and/or a "blowdown tracer" as these terms are used and defined herein. The
term "inert tracer", as used herein, refers to only tracers that are inert, as defined
below. The term "active tracer", as used herein, refers to tracers that provide some
performance benefit downstream of the sample point and/or are consumed or have their
tracer characteristic changed by chemical alteration to some extent within the boiler
system downstream of the sample point. A feedwater tracer is any tracer monitored
in the boiler feedwater upstream of the feedwater inlet to the boiler for any purpose
of the present invention, and it may be an inert tracer or an active tracer. A phosphate-feed
tracer is a tracer monitored in any stream containing the phosphate feeding to the
boiler (which stream often but not necessarily is the feedwater stream downstream
of phosphate pump) for the purpose of monitoring the phosphate feed rate. The phosphate-feed
tracer may be an inert tracer or an active tracer, and it may also be the feedwater
tracer. The phosphate-feed tracer of course is present in the stream in which it is
monitored in known proportion to the phosphate so that its concentration can be correlated
to the phosphate concentration. A blowdown tracer is a tracer monitored in the blowdown
for the purpose of determining boiler cycles. The blowdown tracer must be inert, and
it may also be the feedwater tracer and/or the phosphate-feed tracer. Thus a single
inert tracer may optionally be the feedwater, phosphate-feed and blowdown tracer,
while an active tracer can only be the feedwater and/or the phosphate-feed tracer.
A single inert tracer that acts as a feedwater, phosphate feed and blowdown tracer
is referred to herein as the solitary tracer.
[0070] When a tracer(s) is employed for the primary and/or sole purpose of being a feedwater
and/or phosphate-feed tracer for the present process, it may or may not be an inert
tracer, as discussed above. If a tracer is not inert, another, and inert, tracer(s)
is also employed. In other words, at least one inert blowdown tracer must be used
to monitor the boiler cycles, and such inert blowdown tracer is preferably, but not
necessarily, also the feedwater tracer and/or phosphate-feed tracer.
[0071] A phosphate-feed or solitary tracer would typically be charged to a water system
together or concomitantly with the phosphate treatment agent. Such tracer(s) may be
a component of a formulated phosphate product, and in most instances it would be a
compound(s) added to the formulated phosphate product, or otherwise added to the water
system in proportion to the phosphate. After introduction to the boiler it would not
precisely follow the phosphate in the system, and instead it can then act as a blowdown
tracer for the boiler-cycles determination purposes of the present invention. Given
the capability of an inert tracer to function as the phosphate-feed tracer, as a feedwater
tracer, and as the blowdown tracer for boiler cycles determination purposes, there
would often be no benefit in, or practical reason for, the use of another tracer.
Nonetheless the use of two or three inert tracers separately functioning as phosphate-feed,
feedwater and/or blowdown tracers is not excluded from the process of the present
invention, and in some instances there may be a benefit in, or practical reason for,
the use of both or all three.
[0072] The inert tracer(s) must be transportable with the water of the boiler system (except
of course the water discharged from the boiler as steam) and thus wholly water-soluble
therein at the concentration it is used, under the temperature and pressure conditions
to be encountered. Preferably the selected inert tracer(s) also meets the following
criteria:
1. Be thermally stable and not decompose at the temperature within a boiler;
2. Be detectable on a continuous or semicontinuous basis and susceptible to concentration
measurements that are accurate, repeatable and capable of being performed on feedwater
and blowdown water;
3. Be substantially foreign to the chemical species that are normally present in the
water of the boiler systems in which the inert tracer(s) may be used;
4. Be substantially impervious to interference from, or biasing by, the chemical species
that are normally present in the water of the boiler systems in which the inert tracer(s)
may be used;
5. Be substantially impervious to any of its own potential specific or selective losses
from the water of the boiler system, including selective carry-over;
6. Be compatible with all treatment agents employed in the water of the boiler systems
in which the inert tracer(s) may be used, and thus in no way reduce the efficacy thereof;
7. Be compatible with all components of its formulation and such compatibility preferably
should endure despite the required concentrations of the tracer(s) and/or other components
in such a formulation, and despite the storage and/or transportation conditions encountered;
and
8. Be reasonably nontoxic and environmentally safe, not only within the environs of
the water of the boiler system in which it may be used, but also upon discharge therefrom.
[0073] In preferred embodiment, the chemical compound(s) selected as an inert tracer(s)
should not be one that is consumed or lost to the water of the boiler system, for
instance due to degradation, deposition, complexation, or other phenomena, unless
such consumption or loss is at a rate that is predictable. The inert tracer(s) used
in the present invention is preferably substantially unconsumed in the use environment.
An inert tracer(s) that is wholly inert in the water-system environment would not
react with any of the components in the water of the boiler system to which it is
added, would not degrade in the environment of the water of the boiler system, would
be incapable of coupling and/or depositing in any manner within such boiler system
and would not appreciably be effected by other system parameters such as metallurgical
composition, heat changes or heat content. There are water-soluble inert tracer(s)
that are wholly inert, or substantially inert, in the aqueous environments likely
to be encountered in industrial boiler systems. Further, it is believed that an inert
tracer(s) having a degree of inertness such that no more than 10 weight percent thereof
is lost due to reaction, degradation, coupling and/or deposition during the time that
elapses between its addition and its discharge as a blowdown component is sufficiently,
or substantially, inert for the purpose of the present invention for most, if not
all, tracer(s) monitorings.
[0074] Generally it is desirable to employ the least amount of inert and/or active tracer(s)
that is practical for the circumstance, and the amount of the inert and/or active
tracer(s) added to the water of the boiler system should be at least an amount effective
for the determinations desired. When one the tracer is an active tracer the minimum
amount thereof that is fed to the water of the boiler system should also be at least
an amount effective for the desired activity thereof is such effective amount is greater
than that required for the determinations desired. Seldom would an inert and/or active
tracer(s) be deliberately fed to the water of a boiler system in an amount grossly
in excess of the minimum effective amount because there generally would be no practical
purpose in doing so that would justify the costs involved and any deleterious effects
on the quality of the water of the boiler caused by the presence of the inert and/or
active tracer(s) therein.
[0075] The amount of inert and/or active tracer(s) to be added to the water of the boiler
system that is effective without being grossly excessive will vary with a variety
of factors including, without limitation, the inert and/or active tracer(s) and monitoring
method selected, the potential for background interference with the selected monitoring
method, the magnitude of the expected inert and/or active tracer(s) concentration
in the feedwater and/or blowdown, the monitoring mode (which generally would be an
on-line continuous monitoring mode), and other similar factors. Generally the dosage
of an inert tracer(s) to a water of the boiler system will be at least sufficient
to provide a concentration of tracer(s) in the blowdown at steady state of at least
about 0.1 ppb, and more commonly at least about 5 ppb or higher, up to about 100 or
200 ppm, in the blowdown.
[0076] By the terms "tracing" and "monitoring" are meant herein, unless expressly indicated
otherwise, the determination of the concentration of the inert and/or active tracer(s)
in the feedwater and/or blowdown. Such tracing/monitoring would seldom be conducted
on a singular, intermittent or semi-continuous basis for the purpose of the present
invention, but instead on a substantially continuous basis, and preferably the concentration
determination is conducted on-site (at the site of the boiler system) to provide a
rapid real-time determination.
[0077] The blowdown tracer(s) preferably is added to the water of the boiler system in known
proportion to the feedwater, and preferably the blowdown tracer(s) is introduced into
the boiler system together with the feedwater at a known and constant concentration
therein.
[0078] The tracer(s) formulation, or "product", may be an aqueous solution or other substantially
homogeneous admixture that disperses with reasonable rapidity in the system to which
it is added. Since in most any instance the tracer(s) would be added to a boiler system
as a component of a formulation, rather than as dry solid or neat liquid, the tracer(s)'s
concentration may be correlated not to the numerical concentration value of the tracer(s)
itself but instead to the concentration of a product, which in turn can be correlated
to the concentration of the inert tracer(s) when and if such information is required.
[0079] Among the substantially boiler-system-inert fluorescent compounds are the mono-,
di- and trisulfonated naphthalenes, including their water-soluble salts, particularly
the various naphthalene mono- and disulfonic acid isomers, which are a preferred inert
tracer(s) for use in the present invention. The naphthalene mono- and disulfonic acid
isomers are water-soluble, generally available commercially and easily detectable
and quantifiable by known fluorescence analysis techniques. Preferred naphthalene
mono- and disulfonic acid isomers are the water-soluble salts of naphthalene sulfonic
acid ("NSA"), such as 2-NSA, and naphthalene disulfonic acid ("NDSA" or "NDA"), for
instance 1,5-NDSA. Many of these inert tracer(s) (mono-, di- and trisulfonated naphthalenes
and mixtures thereof) are extremely compatible with the environments of most boiler
systems. Among these preferred fluorescent tracers, 2-NSA and (1,5-NDSA) have been
found to be thermally stable (substantially inert) at temperatures up to at least
about 540 °C (1004 °F), for at least 24 hours at 285 °C (545 °F) and at pressures
up to about 1,500 psig for time periods commensurate with commercial boiler holding
times. Such inert fluorescent tracers have been found to carryover into the steam
discharged from commercial boilers at concentrations of less than 500 ppt when present
in the boiler waters at concentrations within the range of from about 5 to 10 ppm,
and thus these tracers are not selectively carried over into the steam, and do not
carry over into the steam in any appreciable amount. In addition, it has been found
that the contribution to conductivity of the mono-, di- and trisulfonated naphthalenes
is minimal at the ppb levels used for fluorescence determination in either the boiler
feedwater or the blowdown.
[0080] Another group of inert fluorescent tracers that are preferred for use in the process
of the present invention, particularly under pressures of no more than about 1,000
psi, are the various sulfonated derivatives of pyrene, such as 1,3,6,8-pyrene tetrasulfonic
acid, and the various water-soluble salts of such sulfonated pyrene derivatives.
[0081] The inert and/or active tracer(s) is preferably selected from among those that are
easily quantifiable by a fluorescence analysis method, a preferred analytical technique
for the purposes of the present process. Other analysis methods not excluded for use
in quantifying the inert and/or active tracer(s) are HPLC and fluorescence analysis
combinations, which are described in more detail below.
[0082] An active tracer may be any chemical specie that is sufficiently water soluble and
can be monitored in the feedwater and/or phosphate with sufficient ease for the purposes
of the present invention. Active tracer chemicals may be lost to the boiler system
due to both selective and nonselective mechanisms. In comparison, inert tracers are
lost to the boiler system substantially only due to nonselective mechanisms such as
removal as part of the blowdown. For the purposes of the present invention, an active
tracer used as a feedwater and/or phosphate-feed tracer may be selectively lost to
the boiler water in any amount by any mechanism, provided that such selective loss
occurs downstream of the monitoring for such active tracer. An active tracer may have
no role in boiler water chemistry other than its function as a feedwater and/or phosphate-feed
tracer, but preferably it also functions as an operative chemical species within the
boiler system.
[0083] An active tracer may be, for instance, a corrosion inhibitor. A corrosion control
program usually depends on specific inhibitors to minimize the anodic or cathodic
electrochemical corrosion reaction, or both. Among the various types of corrosion
inhibitors are organic compounds, which act by adsorbing or chemisorbing as thin layers
on metal surfaces to separate the water and metal. These materials form and maintain
a dynamic barrier between the water and metal phases to prevent corrosion. One series
of compounds applied to reduce copper and copper-alloy corrosion are aromatic organic
corrosion inhibitors. This series of organic compounds, which includes mercaptobenzothiazole
("MBT"), benzotriazole ("BT"), butylbenzotriazole ("BBT"), tolyltriazole ("TT"), naphthotriazole
("NTA") and related compounds, react with the metal surface and form protective films
on copper and copper alloys. These compounds are active corrosion inhibition treatment
components and are referred to generally herein as copper corrosion inhibitors or
corrosion inhibitors, or as aromatic azoles, and at times as triazoles or aromatic(thio)(tri)azoles.
The conventional analytical procedure for analysis of copper corrosion inhibitors
is a UV (ultraviolet light) photolysis/photometric method, and is well known to persons
of ordinary skill in the art. This method, however, has the a number of limitations.
It is not well suited for continuous monitoring and/or control. It provides results
that are strongly dependent upon the operator's laboratory technique. It cannot distinguish
the chemical structure of the aromatic(thio)(tri)azole that is present. Its observed
response is non-linear with respect to aromatic(thio)(tri)azole dosage. It requires
that the aromatic(thio)(tri)azoles be degraded with an ultraviolet lamp in the presence
of a color-forming reagent. Therefore the preferred analytical technique for aromatic(thio)(tri)azoles
when used as an active tracer in the process of the present invention is fluorescence
emission spectroscopy discussed in more detail below. The use of fluorescence emission
spectroscopy for the analysis of aromatic(thio)(tri)azoles is discussed in more detail
in U.S. Patent Application Serial No. 07/872,624, filed on April 22, 1992, incorporated
hereinto by reference.
Fluorescence Emission Spectroscopy
[0084] The detection and quantification of specific substances by fluorescence emission
spectroscopy is founded upon the proportionality between the amount of emitted light
and the amount of a fluoresced substance present. When energy in the form of light,
including ultra violet and visible light, is directed into a sample cell, fluorescent
substances therein will absorb the energy and then emit that energy as light having
a longer wavelength than the absorbed light. A fluorescing molecule absorbs a photon
resulting in the promotion of an electron from the ground energy state to an excited
state. When the electron's excited state relaxes from a higher energy vibrationally-excited
state to the lowest energy vibrationally-excited state, energy is lost in the form
of heat. When the electron relaxes to the ground electronic state, light is emitted
at a lower energy than that absorbed due to the heat-energy loss, and hence at a longer
wavelength than the absorption. The amount of emitted light is determined by a photodetector.
In practice, the light is directed into the sample cell through an optical light filter
so that the light transmitted is of a known wavelength, which is referred to as the
excitation wavelength and generally reported in nanometers ("nm"). The emitted light
is similarly screened through a filter so that the amount of emitted light is measured
at a known wavelength or a spectrum of wavelengths, which is referred to as the emission
wavelength and generally also reported in nanometers. When the measurement of specific
substances or categories of substances at low concentrations is desired or required,
such as often is the case for the process of the present invention, the filters are
set for a specific combination of excitation and emission wavelengths, selected for
substantially optimum low-level measurements.
[0085] In general, the concentration of a tracer(s) can be determined from a comparison
of a sample's emissions intensity to a calibration curve of the given tracer's concentration
versus emissions, for the same set of excitation wavelength/emission wavelengths.
Such a concentration-by-comparison method by which the sensed emissions are converted
to a concentration equivalent preferably is employed to determine concentrations of
a tracer(s) that are within the concentration range over which a linear emission response
is observed, and this concentration range is referred to herein as the "linear-emission-response
concentration range". The linear-emission-response concentration range is to some
extent dependent upon the specific tracer(s) and the excitation wavelength/emission
wavelength set employed. At tracer concentrations higher than a given tracer(s)'s
linear-emission-response concentration range, there is a negative deviation from ideal
(linear) behavior, the degree of emission for a given concentration being less than
predicted by a linear extrapolation. In such instances, the sample can be diluted
by known factors until the concentration of the tracer(s) therein falls within the
linear-emission-response concentration range. If the tracer(s) is present in the sample
at only very low concentrations, there are techniques for concentrating the tracer(s)
by known factors until its concentration falls within the linear-emission-response
concentration range or is otherwise more readily measured, for instance by liquid-liquid
extraction. Nonetheless, preferably a calibration curve over the linear-emission-response
concentration range would be prepared or obtained before employing a given tracer(s),
and preferably the tracer(s) would be added to the feedwater of the boiler system
in an amount sufficient to provide a concentration of the tracer(s) in the feedwater
and/or boiler that is within the linear-emission-response concentration range. Generally
the linear-emission-response concentration range of a tracer(s) is sufficiently broad
to readily estimate the amount of the tracer(s) that will be sufficient for this purpose.
A linear-emission-response concentration range will most often extend through a concentration
range from a concentration of "m" to a concentration of at least 10m.
[0086] A determination of the presence of a fluorescent tracer(s) and preferably the concentration
thereof in the feedwater and/or blowdown from a boiler system can be made when the
concentration of the tracer(s) in the feedwater and/or boiler is only several parts
per million (ppm) or even parts per billion (ppb) for some of the tracer(s) that can
be employed in the process of the present invention. In preferred embodiment, the
amount of a fluorescent tracer(s) added to the feedwater and/or boiler system should
be sufficient to provide a concentration of the tracer(s) in the feedwater and/or
blowdown to be analyzed of from about 5 ppb to about 100 or 200 ppm, although the
preferred inert tracers specifically mentioned herein need not be present in the sample
analyzed in excess of about 5 or 7 ppm. Such analyses, that is, the measurements of
the light emitted in response to the light transmitted to the feedwater and/or blowdown,
can be made on-site, preferably on an almost instant and continuous basis, with simple
portable equipment, such as the photodetector and screens described above.
[0087] At times it may be desired to employ a plurality of tracers for the reasons mentioned
above and other reasons. For instance, it may be desired to use a plurality of tracers
to confirm that neither is undergoing any tracer-specific loss or one tracer to detect
a given variance and another for the detection of a different variance or other parameter.
Such separate and distinct tracers can each be detected and quantified in a single
feedwater and/or blowdown fraction despite both being fluorescent tracers if their
respective wavelengths of emission do not interfere with one another. Thus concurrent
analyses for multiple tracers is possible by selection of tracers having appropriate
spectral characteristics. Preferably widely separated wavelengths of radiation should
be used to excite each of the tracers and their fluorescent emissions should be observed
and measured at widely separated emission wavelengths. A separate concentration calibration
curve may be prepared or obtained for each tracer. In other words, more than one tracer
can be employed, and then the presence and/or concentration of each such tracer in
the feedwater and/or boiler system should be determined using analytical parameters
(particularly the excitation/emission wavelengths) effective for each such tracer,
which analytical parameters preferably are sufficiently distinct to differentiate
between measurements.
[0088] Fluorescence emission spectroscopy on a substantially continuous basis, at least
over a given time period, is one of the preferred analytical techniques for the process
of the present invention. It is one of the preferred analysis techniques for quantifying
and determining the concentration of the tracer(s) in a boiler system and it is an
analysis technique having significant advantages. Fluorescent chemical tracers and
monitoring techniques are now known, for instance as disclosed in U.S. Patent No.
4,783,314, J. E. Hoots and B. E. Hunt, issued November 8, 1988, incorporated herein
by reference, wherein inert fluorescent tracers are employed in combination with a
fluorescence monitoring, such as the sodium salt of 2-naphthalenesulfonic acid.
[0089] When the tracer is 2-NSA, one of the water-soluble salts of naphthalene sulfonic
acid ("NSA"), its concentration in the feedwater and/or blowdown from a boiler system
can be fluorometrically measured by excitation at 277 nm and emission measurement
at 334 nm, and the emissions observed referenced to a standard aqueous solution containing
0.5 ppm 2-NSA, as acid actives.
[0090] A Gilford Fluoro IV dual-monochromator spectrofluorometer can be used for a fluorometric
analysis conducted on an intermittent basis and for on-line fluorescence monitoring,
a portable fluorometer equipped with appropriate excitation and emission filters and
a quartz flow through cell can be used, such as is commercially available from Turner
Designs (Sunnyvale, California) Model Fluorometer 10 AU, which is mentioned above.
[0091] In general for most fluorescence emission spectroscopy methods having a reasonable
degree of practicality, it is preferable to perform the analysis without isolating
in any manner the tracer(s). Thus there may be some degree of background fluorescence
in the feedwater and/or blowdown on which the fluorescence analysis is conducted,
which background fluorescence may come from chemical compounds in the boiler system
(including the feedwater system thereof) that are unrelated to the present process.
In instances where the background fluorescence is low, the relative intensities (measured
against a standard fluorescent compound at a standard concentration and assigned a
relative intensity for instance 100) of the fluorescence of the tracer versus the
background can be very high, for instance a ratio of 100/10 or 500/10 when certain
combinations of excitation and emission wavelengths are employed even at low fluorescent
compound concentrations, and such ratios would be representative of a "relative fluorescence"
(under like conditions) of respectively 10 and 50. In preferred embodiment, the excitation/emission
wavelengths and/or the amount of tracer employed are selected to provide a relative
fluorescence of at least about 5 or 10 for the given background fluorescence anticipated.
[0092] For instance, for most feedwater and/or boiler water backgrounds, a compound that
has a relative fluorescence of at least about 5 at a reasonable concentration is very
suitable as a tracer(s). When there is or may be a specific chemical specie of reasonably
high fluorescence in the background, the tracer(s) and/or the excitation and/or emission
wavelengths often can be selected to nullify or at least minimize any interference
of the tracer measurement(s) caused by the presence of such specie.
[0093] One method for the continuous on-stream monitoring of chemical tracers by fluorescence
emission spectroscopy and other analysis methods is described in U.S. Patent No. 4,992,380,
B. E. Moriarity, J. J. Hickey, W. H. Hoy, J. E. Hoots and D. A. Johnson, issued February
12, 1991, incorporated hereinto by reference.
Combined HPLC-Fluorescence Analysis
[0094] The combination of high-pressure liquid chromatography ("HPLC") and fluorescence
analyses of fluorescent tracers is a powerful tool for the present process, particularly
when very low levels of the tracer(s) are used or the background fluorescence encountered
would otherwise interfere with the efficacy of the fluorescence analysis. The HPLC-fluorescence
analysis method allows the tracer compound(s) to be separated from the fluid matrix
and then the tracer concentration(s) can be measured. The combination of HPLC-fluorescence
analysis is particularly effective for measuring minute levels of the tracer(s) in
highly contaminated fluids.
[0095] The HPLC method can also be effectively employed to separate a tracer compound(s)
from a fluid matrix for the purposes of then employing a tracer-detection method(s)
other than the fluorescence analysis, and such other tracer-detection methods include
without limitation light absorbance, post-column derivatization, conductivity and
the like, which methods are described in "Techniques in Liquid Chromatography", C.
F. Simpson ed., John Wiley & Sons, New York, pp. 121-122, 1982, incorporated hereinto
by reference, and "Standard Method For The Examination Of Water And Wastewater", 17th
Edition, American Public Health Association, pp. 6-9 to 6-10, 1989, incorporated hereinto
by reference.
[0096] Analytical techniques for quantifying the presence and/or concentration of a chemical
specie without isolation thereof are within an evolving technology, and the above
survey of reasonable analytical techniques for use in monitoring the tracer(s) in
the process of the present invention may presently not even be exhaustive, and most
likely techniques equivalent to the above for the purposes of the present invention
will be developed in the future.
[0097] A tracer(s) may be selected for a given process based on a preference for one or
more analytical techniques, or an analytical technique may be selected for a given
process based on a preference for one or more tracers.
[0098] As noted above, in preferred embodiment, the chemical compound(s) selected as the
tracer(s) is soluble in the feedwater and/or boiler system to which it is added to
the concentration value desired and is substantially stable in the environment thereof
for the useful life expected of the tracer(s), particularly since it is desired not
merely to detect the presence of some amount of the tracer(s), but also to determine
the concentration thereof, or change in concentration. In preferred embodiment, the
combination of the chemical compound(s) selected as the tracer(s) and the analytical
technique selected for determining the presence of such tracer(s), permits such determination
without isolation of the tracer(s), and more preferably should permit such determination
on a continuous and/or on-line basis.
[0099] As discussed above, the phosphate-feed tracer in some preferred embodiments is added
to the feedwater as a component of a formulated treatment product, particularly a
formulated treatment product that also contains the phosphate being added to the feedwater.
Formulating the tracer and the phosphate together is one technique that easily will
provide a reliable correlation between the tracer feed rate and the phosphate feed
rate. Such a formulated treatment product may also contain other treatment components,
for instance dispersants and other treatment additives. For example, polymeric dispersants
are often added to boilers to control iron deposits therein. Such dispersants include
without limitation polyacrylic acid, polymethacrylic acid, acrylic acid/acrylamide
copolymers, polyisopropenyl phosphonic acid and the water-soluble salts of such polymeric
species, and these and other boiler additives are conveniently injected into the feedwater
of a boiler system together with phosphate as a formulated treatment product. Such
formulated treatment products routinely contain a diluent for the active components,
which diluent can be water. The presence of such other boiler treatment additives
in the formulation that contains one or more tracers is an embodiment, and at times
a preferred embodiment of the present invention.
[0100] Although high pressure boilers and the requirements thereof are at times discussed
above in relation to the present invention and pH/phosphate programs, neither the
use of pH/phosphate programs nor the use of the present invention are limited to high
pressure boilers. Instead, pH/phosphate programs and the present invention are also
useful for boilers having lower operating pressures, for instance boilers having operating
pressures of at least about 15 psi.
[0101] The above descriptions of boiler systems, pH/phosphate programs and other treatment
additives for boilers are not exhaustive, and further descriptions are available in
the literature in the boiler field, including in "The Chemical Treatment Of Boiler
Water", James W. McCoy, pages 65-68, 81-87 and 131-139, 1981, 2nd printing 1984, Chemical
Publishing Co., Inc., New York, N.Y., incorporated hereinto by reference, "The Nalco
Guide To Boiler Failure Analysis", Robert D. Port and Harvey M. Herro, McGraw-Hill,
Inc., New York, N.Y., incorporated hereinto by reference, "Steam - Its Generation
And Use", 40th Edition, S. C. Stultz and J. B. Kitto ed., Babcock & Wilcox, a McDermott
company, Barberton, Ohio, 1992, incorporated hereinto by reference, and "The Nalco
Water Handbook", Second Edition, F. N. Kemmer ed., McGraw-Hill Company, New York,
N.Y., 1988, particularly pp. 34.1-24.21, incorporated hereinto by reference.
[0102] The present invention is a method of controlling a pH/phosphate program in a boiler
water system wherein
steam is generated from boiler water within a boiler and the steam is discharged
from the boiler,
the boiler water contains impurities and the concentration of the impurities in
the boiler water is reduced by discharging fractions of the boiler water as blowdown,
feedwater is fed to the boiler to replenish the water discharged from the boiler
as steam and blowdown,
wherein the boiler has a concentration cycle value which is the value of the average
concentration ("C
F") of an inert component in the blowdown at steady state divided by its concentration
("C
I") in the feedwater,
and phosphate and sodium hydroxide or other alkalinity source are fed to the boiler
in amounts to attempt to provide a sodium to phosphate mole ratio in the boiler water
within a target mole ratio range that is bounded by known PO₄/pH coordinates on a
plot of orthophosphate concentration versus pH of a pH/phosphate program, comprising:
(1) determining the feed rate of the phosphate to the boiler by adding to a phosphate-feed
stream containing the phosphate feeding to the boiler a phosphate-feed tracer in known
proportion to the phosphate, determining the concentration of the phosphate-feed tracer
in the phosphate-feed stream id correlating the phosphate-feed tracer concentration
to the concentration of the phosphate;
(2) determining the boiler cycles value by adding an inert blowdown tracer to the
boiler water at a known rate and determining the concentration of the blowdown tracer
in the blowdown;
(3) determining the blowdown pH value;
(4) optionally determining the feed rate of the alkalinity source to the boiler by
sampling a alkalinity source-feed stream containing the alkalinity source feeding
to the boiler and determining the concentration of the alkalinity source in the alkalinity
source-feed stream;
(5) providing the data defining at least the phosphate-feed rate value, the boiler
cycles value, and the blowdown pH value to a control center;
(6) optionally maintaining the alkalinity source feed rate constant or providing the
data defining the alkalinity source feed rate to the control center;
wherein the control center directs at least one responsive adjustment of the phosphate
feed to the boiler, the alkalinity source feed to the boiler, and/or the blowdown
rate when a sodium to phosphate mole ratio in the boiler water strays from the target
mole ratio,
wherein the responsive adjustment directed by the control center changes the sodium
to phosphate mole ratio in the boiler water in the direction of regaining the target
sodium to phosphate mole ratio in the boiler water.
[0103] In preferred embodiment, the phosphate is fed to the feedwater stream by a phosphate
pump upstream of the boiler and the phosphate-feed stream is the feedwater stream
downstream of the phosphate pump.
[0104] In some preferred embodiments, a solitary tracer is both the phosphate-feed tracer
and the blowdown tracer. In further preferred embodiment, the solitary tracer is also
a feedwater tracer and the concentration of the solitary tracer in the feedwater stream
is correlated to the feed rate of the feedwater to the boiler.
[0105] In preferred embodiment, the phosphate-feed tracer is a fluorescent tracer and the
concentration of the phosphate-feed tracer in the phosphate-feed stream is substantially
continuously monitored on-line by sensing the fluorescent characteristic of the phosphate-feed
tracer in the phosphate-feed stream. In preferred embodiment, the blowdown tracer
is a fluorescent tracer and the boiler cycles is substantially continuously monitored
on-line by sensing the fluorescent characteristic of the blowdown tracer in the blowdown.
[0106] In preferred embodiment, the phosphate-feed tracer and the blowdown tracer are distinct
chemical species. In further preferred embodiment, the phosphate-feed tracer and the
blowdown tracer are each fluorescent tracers and are distinct chemical species, wherein
the phosphate-feed tracer and the blowdown tracer are each detected and quantified
in a single feedwater and/or blowdown fraction by concurrent fluorescence analyses,
and wherein sufficiently separated wavelengths of radiation are used to excite each
of the phosphate-feed tracer and the blowdown tracer in the single fraction and the
fluorescent emissions of the phosphate-feed tracer and the blowdown tracer are observed
and measured at sufficiently separated emission wavelengths for the concurrent analysis.
In further preferred embodiment, the phosphate-feed tracer and the blowdown tracer
are distinct chemical species, wherein the phosphate-feed tracer and the blowdown
tracer are each detected and quantified in a single feedwater and/or blowdown fraction
by concurrent analyses, and wherein the concurrent analyses each employ a distinct
concentration calibration curve for each respective distinct tracer.
[0107] In preferred embodiment, at least one of the phosphate-feed tracer and the blowdown
tracer is a monosulfonated naphthalene, disulfonated naphthalene or trisulfonated
naphthalene, or a water-soluble salt thereof. In more preferred embodiment, at least
one of the phosphate-feed tracer and the blowdown tracer is a water-soluble salts
of 2-naphthalene sulfonic acid or 1,5-naphthalene disulfonic acid.
[0108] In another preferred embodiment, at least one of the phosphate-feed tracer and the
blowdown tracer is a sulfonated derivative of pyrene. In more preferred embodiment,
at least one of the phosphate-feed tracer and the blowdown tracer is a 1,3,6,8-pyrene
tetrasulfonic acid or water-soluble salt thereof.
[0109] In preferred embodiment, at least one of the phosphate-feed tracer and the blowdown
tracer is a component of a formulated phosphate product.
[0110] In preferred embodiment, a solitary tracer is employed as a inert tracer to function
as the phosphate-feed tracer, as a feedwater tracer, and as the blowdown tracer for
boiler cycles determination purposes.
[0111] In preferred embodiment, a feedwater-tracer is added to the boiler in the feedwater,
wherein the feedwater tracer and the phosphate-feed tracer are the same tracer or
different tracers, and wherein at least one of the phosphate-feed tracer and the feedwater
tracer is an active tracer that is a corrosion inhibitor. In more preferred embodiment,
a feedwater-tracer is added to the boiler in the feedwater, wherein the feedwater
tracer and the phosphate-feed tracer are the same tracer or different tracers, and
wherein at least one of the phosphate-feed tracer and the feedwater tracer is an active
tracer that is an aromatic azole.
[0112] In preferred embodiment, the phosphate-feed tracer is an inert tracer that is added
to the water of the boiler system in an amount at least sufficient to provide a concentration
of the inert tracer in the blowdown at steady state of at least about 0.1 ppm.
[0113] In preferred embodiment, at least one of the phosphate-feed tracer and the blowdown
tracer is a fluorescent tracer and the fluorescent tracer is quantified to provide
the phosphate feed rate value and/or the boiler cycles value by a combination of high-pressure
liquid chromatography to separate the fluorescent tracer from a fluid matrix and fluorescence
analyses of the fluorescent tracer.
[0114] In preferred embodiment, the phosphate-feed tracer is an active tracer, wherein the
active tracer provides a performance benefit to the boiler water system downstream
of the point at which the phosphate feed rate value is determined, and wherein the
phosphate-feed tracer is added to the boiler in at least the amount effective to provide
the performance benefit and at least in the amount effective for the determination
of the phosphate feed rate value.
[0115] In certain preferred embodiments, the tracer selected is not a visible dye, that
is, the tracer is a chemical specie that does not have a strong absorption of electromagnetic
radiation in the visible region, which extends from about 4000 to about 7000 Angstroms
(from about 400 nm to about 700 nm). Such embodiments may be preferred when it is
desirable to maintain a boiler water system free of color.
[0116] Unless expressly indicated otherwise herein, all properties of any chemical compounds,
or compositions containing a plurality of chemical compounds, set forth herein are
such property values as would be determined for such compounds, or compositions, within
the temperature and substantially under atmospheric pressure of the use environment.
[0117] By deposits is meant herein material that forms and/or collects on surfaces of a
boiler system or boiler equipment. By the terminology "__tracer concentration value"
is meant herein the concentration of the tracer in the specified fluid in terms of
weight of the tracer per unit volume of the fluid, or weight of the tracer per unit
weight of the fluid, or some characteristic of the tracer that is proportional to
its concentration in the fluid and can be correlated to a numerical value of the tracer
concentration in the fluid (whether or not that correlation conversion is calculated,
and can be a value of zero or substantially zero (for instance a value that is too
low for detection). Thus the present process includes the detection of the absence
of such chemical species, at least to the limitations of the analytical method employed.
[0118] Unless expressly indicated otherwise herein, the inclusion of a prefix or suffix
in parenthesis designates the word with such prefix or suffix as an alternative. For
instance, "specie(s)" means "specie and/or species", "determination(s)" means "determination
and/or determinations", "tracer(s)" means "tracer and/or tracers", "technique(s)"
means "technique and/or techniques", "chemical(s)" means "chemical and/or chemicals",
"component(s)" means "component and/or components", and the like.
[0119] By "ppm", "ppb" and "ppt" is meant herein respectively "parts per million", "parts
per billion" and "parts per trillion" wherein the "parts" are by weight.
Industrial Applicability of the Invention
[0120] The present invention is applicable to all industries employing a boiler system.